To print an image, an ink jet printer deposits droplets of ink onto a print medium in a desired pattern. The ink is ejected from the nozzles of one or more printheads of the printer. It may be forced out of the nozzles by the activation of a piezoelectric or thermal element mounted in a firing chamber behind each nozzle. As the ink is ejected from a firing chamber through a nozzle, a vacuum is created in the firing chamber. This draws more ink into the chamber from an associated ink channel to ready the chamber for the next firing.
To operate effectively, the printheads need a reliable supply of ink. A number of ink storage and delivery systems have been devised to achieve this. Some of these systems use permanent ink reservoirs that are refilled by a user, whilst others use replaceable ink cartridges. Some of these replaceable ink cartridges incorporate both a printhead and ink reservoir, whilst others include only the reservoir.
In one example, a cartridge has an ink reservoir and a variable-volume pump chamber that links the reservoir with the printhead so as to provide a controlled ink pressure. An actuator on the printer continually presses against a diaphragm of the cartridge pump chamber to pressurise the ink in the chamber. When the chamber needs to be replenished with ink, the actuator pressure is removed so that the diaphragm can move under the influence of an internal spring to expand the chamber volume and draw ink into the chamber from the reservoir. Once the chamber is refilled, actuator pressure is re-applied to re-pressurise the chamber and allow for further printing.
A prior art actuator arrangement is shown in FIG. 1. The actuator 10 is urged upwardly to pressurise a cartridge pump chamber (not shown) by the force of an extension spring 12 acting through a rocker arm 14 that is mounted on a rocker shaft 16. As ink is supplied to the printhead, the amount of ink in the chamber is reduced, and the actuator 10 moves gradually upwards under the spring force. Once the actuator 10 reaches a set height, corresponding to a chamber refill condition, the rocker arm 14 causes an out-of-ink trip flag 18 to break the beam of an optical switch. This indicates a need to refresh the chamber, and a cam 20 is rotated to act on the rocker arm 14 and oppose the force from the spring 12. This removes the actuator pressure from the cartridge pump chamber and allows ink flow from the reservoir to the chamber. After a preset time, sufficient to allow replenishment of the chamber, the cam 20 is further rotated to allow the actuator 10 to again pressurise the cartridge pump chamber.
Other actuator mechanisms are also known, including those described in U.S. Pat. No. 5,856,839 and U.S. Pat. No. 6,550,899, which again provide an actuator and a cam and rocker arrangement. The contents of U.S. Pat. No. 5,856,839 and U.S. Pat. No. 6,550,899 are incorporated herein by reference.